Metallic porous body incorporated by casting into a heat exchanger

ABSTRACT

The present invention relates to a co-cast heat exchanger element intended for a central heating boiler, which heat exchanger element is made from substantially aluminum, the heat exchanger element being provided with walls which enclose a water carrying channel, and with at least one wall which encloses at least one flue gas draft to which a burner can be connected, at least one wall which encloses the at least one flue gas draft being water-cooled in that it also forms a boundary of the water-carrying channel, while one of the water-cooled walls is provided with heat exchanging surface enlarging pins and/or fins which extend in the respective flue gas draft and is also provided with other heat exchanging surface enlarging metallic porous structures.

TECHNICAL FIELD

The present invention relates to a co-cast heat exchanger element intended for a central heating boiler, which heat exchanger element is made from substantially aluminum, the heat exchanger element being provided with walls which enclose a water carrying channel, and with at least one wall which encloses at least one flue gas draft to which a burner can be connected, at least one wall which encloses the at least one flue gas draft being water-cooled in that it also forms a boundary of the water-carrying channel, while at least one of the water-cooled walls is provided with heat exchanging surface enlarging pins and/or fins which extend in the respective flue gas draft and is also provided with other heat exchange surface enlarging metallic porous structures.

The present invention also relates to a method for obtaining such a co-cast heat exchanger element and its use in a central heating boiler.

BACKGROUND ART

A heat exchanger according to above described heat exchanger is known from EP 1722172, wherein the cross-sectional surface of the pins and/or fins is smaller than 25 mm²; the heat exchanger being a mono-casting. Such heat exchanger, with pins with a length of e.g. 15 mm and having a greater surface-content ratio, has a low weight. This results optimally in a thermal inertia of 0.16 kg/kW, which makes the heat exchanger element heating up much more rapidly, thereby reducing the time required for obtaining hot water for domestic use. Such heat exchanger, due to the smaller length of the pins and/or fins, has a smaller cross-section of the flue gas draft. This leads to a higher flow velocity of the flue gases and results in a higher heat transfer coefficient and thus a better efficiency.

The known heat exchanger element is already relatively small for a boiler with such an output. When this boiler is used for heating not only central heating water but also domestic hot water, there is still a need for further improving the compactness, and for a still more rapid heating of the domestic hot water.

WO 02/093644 describes a heat exchanger consisting of open-pore metallic foam as an example of a porous structure, wherein the metallic foam is cast together with structural elements (e.g. water channels) in one single step. The use of such a heat exchanger element on its own in a boiler is not possible as the metallic foam would melt by the heat of the flue gases. On the other hand, casting of complex structures together with the already complex open cell foam body (as an example of a metallic porous structure) is a rather difficult job, resulting in a lot of scrap and waste. Therefore, most people consider connecting an (aluminium) porous body to a heat exchanging element in a separate step. Herein, good heat conducting contact between the porous metal body and the solid metal carrier is indispensable for an efficient functioning of the heat-exchanging devices. This is particularly relevant taking into account the fact that only a small percentage of the solid metal carrier is in contact with the porous metal structure. Establishing excellent thermal/metallic contact can reduce the total dimensions of a heat-exchanging device considerably and thereby reduce material costs and space.

The art already describes a lot of applications of porous metal structures in heat-exchanging devices and the method of attaching the porous metallic structure to a carrier. For example in U.S. Pat. No. 6,397,450 it is stated that direct bonding may be achieved through soldering, active brazing or simply brazing. EP1553379 states that the connection between shell and metal sponge, as an example of a porous metal structure, can be simply made by means of soldering or welding. But the effective quality of these bonds is not always satisfactory.

One can achieve a proper mechanical bond between a porous aluminium body and a solid metal carrier, by, for example soldering, but as this method uses an extra material, heat dissipation from carrier to porous structure, or the other way around can be distorted and the extra material, e.g. Zn, can give corrosion problems at the bonding place and even have the effect of a thermal insulating layer. It also gives an end product which is limited in use for heat applications, i.e. limited by the melting temperature of Zn in the solder.

Sintering, brazing and soldering need working conditions wherein the aluminium-oxides, formed on the surfaces of the porous metallic bodies and the heat exchanging element, have to be removed, e.g. by working in a vacuum oven or by the use of fluxing material. When these aluminium-oxides are not sufficiently removed, no good heat conduction bond can be obtained.

Hence, there is a need for an alternative and easy bonding method that results in a good heat conducting contact between a porous body and a complex solid metal carrier material, wherein heat is easily transported throughout the newly formed structure.

DISCLOSURE OF INVENTION

An aspect of the claimed invention provides a heat exchanger element intended for a central heating boiler having a higher output than the known central heating boilers with comparable dimensions, the intended heat exchanger element being particularly compact and having low weight.

To this end, the heat exchanger element according to the invention is manufactured as a co-casting product from substantially aluminium, the heat exchanger comprising the features of claim 1.

The heat exchanger element has a very flat design, wherein the flue gas draft is wide but not deep (as can be seen in FIG. 1A), which is possible due to the use of shorter and smaller pins and/or fins as heat exchange surface enlarging structures, compared to the ones used in conventional heat exchanger elements for boilers. The use of these pins and/or fins, with their great surface-content ratio and heat exchanging action, makes it possible to cool down the flue gas and to transfer the heat efficiently to the water-cooled walls. The cooling of the long walls in the heat exchanger element is established by a parallel path in a one-piece water channel. This one-piece water channel core renders the manufacture of the core and the positioning of the core in a sand-casting mold relatively simple, so that the manufacture of the heat exchanger element is relatively simple as well and, accordingly, can take place in an economically favourable manner. As is evident for a person skilled in the art, the water-channel core can be built of different pieces put together to form the water-channel core.

Also the incorporation of the metallic porous body into the heat exchanger element is a relatively simple method: this porous body is incorporated in the internal sand core of the heat exchanger. Alternatively, the porous body is built in into the (polystyrene) positive model in a lost foam casting process.

Surprisingly, it was found that the porous metallic body was not affected by the hot molten metal, cast onto the porous metallic body and that a good metallic bond was obtained between the porous metallic body and the cast heat exchanger element. And also that the aluminium-oxides present at the surface of the porous aluminium material did not inhibit a good connection between the porous material and the heat exchanger element. The struts or ligaments of the metallic porous body stay intact into the complete co-cast structure and are properly surrounded by the melt (see FIG. 4) creating a big contact surface, which results in a very good heat transfer and is—by the turbulence enhancing 3-D structure of the porous material—able to extract even more heat from the flue gasses, enhancing further the efficiency of the heat exchanger element.

Hence, with the heat exchanger element according to the invention, a central heating boiler can be made having a greater output than the known central heating boilers with comparable dimensions, while the same or even a better degree of compactness and thermal inertia is achieved.

The heat exchanger element is manufactured as a co-casting, comprising the steps of claim 5, 6 or 7, and can be manufactured in a relatively quick and efficient manner.

According to a further aspect of the invention, intended for increasing the efficiency of a central heating boiler, comprising a heat exchanger according to the invention, each flue gas draft of the heat exchanger may comprise two opposite walls having pins with a cross sectional surface which is smaller than 25 mm².

Another aspect of the invention relates to a central heating boiler comprising at least one heat exchanger element according to the invention.

DEFINITIONS

The heat exchanger element is made from substantially aluminium meaning that the heat exchanger element can be made out of pure aluminium or an aluminium alloy. Wherever in this description is referred to metal, aluminium or one of its alloys is referred to. It should be noted that the terms metal, aluminium and aluminium-alloy will be used throughout this text without meaning anything else than aluminium or one of it's alloys.

The term metallic porous material or body differs from pins and fins in that these metallic porous materials/bodies represent a continuous and complex 3-D structure such as e.g. metallic open cell foam, metallic spacer material, folded knitted or woven metal wire structures or knitted wire mesh. Another distinction between the pins and fins and the metallic porous materials lies in the porosity of these structures. A metallic porous material as used in this text has a porosity of 70% or more.

The term co-casting is explained in claim 5, and can be described in short as a two step casting method, wherein the first casting was performed in the production of the porous metallic body, see e.g. WO 01/14086 or EP1733822; the second or co-casting step being described in this patent application. Co-casting, in the light of this patent application, is also to be understood as casting onto a porous metallic object, thereby obtaining the good metallic bond.

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

FIG. 1 is a perspective view of an exemplary embodiment of a heat exchanger according to the invention.

FIG. 2 is a sectional view taken on the plane II-II′ of FIG. 1.

FIG. 3 is a sectional view taken on the plane III-III′ of FIG. 1.

FIG. 4 is a perspective view of an alternative exemplary embodiment of a heat exchanger according to the invention.

FIG. 5 is a sectional view taken on the plane V-V′ of FIG. 4.

FIG. 6 is a sectional view taken on the plane VI-VI' of FIG. 4.

FIG. 7 is a perspective view of the principle of the parallel water channels of the heat exchanger according to the invention.

FIG. 8 is an optical microscopy picture of a strut of an open cell aluminium foam made of aluminium alloy AlSi7 embedded in the co-cast material of the heat exchanger element made of aluminium alloy AlSi10.

FIG. 9 is a perspective view of an alternative embodiment of the present invention.

FIG. 10 is a sectional view taken on the plane X-X′ of FIG. 9.

FIG. 11 is a sectional view taken on the plane XI-XI′ of FIG. 9.

FIG. 12 shows a perspective view of the water channel used in FIG. 9.

REFERENCE NUMBERS LIST

-   -   1 heat exchanger element     -   2 walls     -   3, 30 water-carrying channel     -   4 core of water-channel     -   5 burner     -   6 combustion chamber     -   7 flue gas draft     -   8 fins     -   9 pins     -   10 porous metallic body     -   11, 110 inlet     -   12, 120 outlet     -   13 long wall of the heat exchanging element 1     -   14 short wall of the heat exchanging element 1     -   15 part of strut of open cell aluminium foam     -   16 part of carrier material cast onto the open cell aluminium         foam     -   17 fins in water carrying channel     -   A first part of flue gas draft 7     -   B second part of flue gas draft 7     -   C third part of flue gas draft 7

MODE(S) FOR CARRYING OUT THE INVENTION

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention.

FIGS. 1, 2 and 3 show an exemplary embodiment of the heat exchanger 1 according to the invention. Heat exchanger 1 is manufactured as a co-casting substantially from aluminium. The heat exchanger comprises a number of walls 2, which walls enclose on one side a water carrying channel 3 and on the other side a flue gas draft 7. The flue gas draft 7 extends from the burner space 6. The burner space 6 is intended for accommodating a burner. Preferably, the burner is a metal fiber burner membrane, as described in WO 2004/092647. The walls 2 enclosing the flue gas draft 7 on the long walls 13 of the heat exchanging element are water cooled by the water carrying channel 3. The water carrying channel 3 is of such design that it forms two parallel water channels, one on each long wall 13 with respect to the burner space 6 and flue gas draft 7, as shown in FIG. 7. The water-carrying channel 3 is, preferably, of such design that it is formed with a core 4, as shown in FIG. 7. The flue gas draft comprises two opposite walls 2 (i.e. long walls 13) having in the upper part A fins 8 extending substantially perpendicular thereto, which fins enlarge the heat exchanging surface and extend into the flue gas draft 7. Part B of the long walls 13 comprise pins 9, also extending substantially perpendicular to the wall 13 and enlarging the heat exchanging surface. Preferably, the pins have a cross-sectional surface which is smaller than 25 mm² and a length of approx. 15 mm. Part C of the long walls 13 comprises a metallic porous structure for enlarging the heat exchanging surface; but also for extracting more energy out of the flue gases by the turbulence enhancing 3D-architecture of the metallic porous body. The use of such a porous metallic body is most effective in the lower temperature ranges of the flue gases and accordingly in a heat exchanger element of this type in the lower regions of the heat exchanger element. The flow speed of the flue gases gets lower and lower and the temperature difference with the cooling water (i.e. the water to be heated) is also very small, making the heat exchange dependant on the heat exchange surface enlarging structure or body.

Preferably, the metallic porous material is an open cell aluminium foam, e.g. as described in WO 01/14086. Using an open cell aluminium foam (completely filled flue gas draft) with a 5 mm cell diameter, and 700 μm strut thickness, having a porosity of 90%, gives a 20% better heat transfer than the same surface provided with pins of 4 mm diameter (e.g. as used in part B and having a porosity of 60%). This better heat transfer can be translated in a reduction of the heat exchanger surface, and also the weight of the heat exchanger element, by 20% and result in a more compact heat exchanger element 1 or, in other words, gives possibilities to miniaturise the heat exchanger element.

In another preferred embodiment, the metallic porous material is a metallic spacer material, e.g. as described in EP1733822.

The flow system of the water carrying channel in FIG. 7 can be considered to be a parallel connection. Water coming from a central heating pipe system enters the heat exchanger adjacent its bottom side at the location of arrow 11. From here, the water enters the feed-in part 3 a of the water carrying channel. The channel 3 a divides into two separate channel parts 3 b and 3 c. The water divides in these two channels 3 b and 3 c after which the water flows into the common channel 3 d, thereafter the water leaves the heat exchanger via outlet 12. With such embodiment of the water-carrying channel 3, it is effected that only the long walls 13 of the flue gas draft 7 and the burner space 6 are water-cooled. Because of the dimensioning of the heat exchanger element 1 resulting in a very flat heat exchanger element, and the heat exchanger element being cooled in a very efficient manner, it does not need cooling on the short walls 14 of the heat exchanger element and makes the heat exchanger element very compact. The optimal heat transfer to the water to be heated, makes that the heat exchanger element nowhere becomes overheated, thus an optimal efficiency is obtained and all parts of the heat exchanger remain sufficiently cooled. Preferably, both parallel channels of the water carrying channel are provided with surface enlarging pins on their inner side, for further enhancing the heat transfer from the metal of the heat exchanger element to the water to be heated.

In the present exemplary embodiments of FIGS. 1 and 4, the flue gases flow from the top to the bottom through the flue gas draft 7, and the water to be heated flows from the bottom to the top, as described above.

The heat exchanger element 1 is preferably manufactured by means of a casting process, such as, for instance, sand casting or die-casting. Preferably, use is then made of at least one core to form the water channel and at least one second core for forming the flue gas channel(s). These flue gas draft cores comprising the metallic porous structures. Alternatively, also a lost foam casting process can be used. The metallic porous body sand core is than build in into the (polystyrene) foam positive model. Alternatively, in lost foam casting, the metallic porous body can be build in into the (polystyrene) foam positive model, The metallic porous body will than be filled with the sand used for the lost foam casting, and no separate step for making a sand core is necessary.

The heat exchanger 1 of FIGS. 1 and 4 are produced by the sand co-casting process. First a piece of an aluminium porous body is put in a core box. A mixture of sand and binder is then blown into the void space in the core box, thereby obtaining a hybrid body of metallic porous body filled with the sand-binder mix. The sand-binder mix is hardened thereby obtaining a metallic porous body—sand core. Thereafter the core box is removed. The metallic porous body—sand core is then integrated into a flue gas draft sand core, which is placed in a moulding box together with the water channel core 4. The molten metal is poured into the moulding and after the necessary cooling down, the sand core is removed. This results in the heat exchanger element 1 as depicted in FIG. 1 or 4.

In an alternative embodiment, the heat exchanger element 1 is made via a lost foam co-casting method. Here, the production of a metallic porous body containing heat exchanger element comprises following steps. First, a metallic porous body-sand core, obtained as in paragraph 30, is build in into a polystyrene pattern (or positive) of the heat exchanger element and further prepared as known in the art. The “polystyrene pattern—metallic porous body-sand core” hybrid cluster is placed into the casting flask and backed-up with un-bonded sand. After the mold compaction, the polystyrene pattern is poured with the molten metal. Then only a relative simple filter action is needed to remove the un-bonded sand from around, and out of, the cast heat exchanger element. And also the sand of the metallic porous body-sand core needs to be removed. Alternatively, the metallic porous body is built into the polystyrene pattern of the heat exchanger element. Then also the metallic porous body will be backed up with unbonded sand, which will be easily removed after co-casting of the heat exchanger element

Part A of the heat exchanger element, in FIGS. 1 and 4, is designed in a tulip form for obtaining low NOx and low CO emissions. This is mainly obtained by the specific form of the flue gas draft part A and the long fins 8 removing already a lot of the heat from the flue gases. This tulip form seems particularly useful when using a burner of the type as described in WO 2004/092647. The tulip-like form can be described as follows: the burner chamber is bound by the metallic burner 6, thereafter the flue gas draft 7 widens and thereafter narrows. This specific form is especially designed to follow the flame pattern and it bends the flames equally without abrupt altering of the flame. This creates enough space for a proper combustion, thereby reaching low emissions of NOx and CO and thereby also attaining a very compact design.

FIG. 4 shows an alternative embodiment of the invention. Same reference numbers describe same structures as in FIG. 1. The embodiment of FIG. 4 is similar to the embodiment in FIG. 1, so only the differences will be explained. As can be seen in FIG. 5, from the third level on (going in the direction of flow of the water to be heated) in the water carrying channel 3, heat exchange surface enlarging ribs are provided. Another difference of the embodiment of FIG. 4 can be found in FIG. 6: here the C-part of the flue gas draft is, next to the metallic porous structure, also containing pins as in part B of the flue gas draft. This modification is an alternative way of integrating a metallic porous body into the heat exchanger element 1, but also other configurations are possible as is evident for the person skilled in the art.

A first worked example embodiment as in FIG. 4, gives a heat exchanger element with an output of approximately 35 kW. The weight of the heat exchanger element per kW to provide, is less than 0.20 kg/kW. In the present exemplary embodiment, the thermal inertia is only 0.17 kg/kW with a compactness of 5.5 kW/l, resulting in a heat exchanger element of 6.0 kg and a volume of 6.4 l. The water carrying channel has a volume of 1.3 litre. The specific load of the burner chamber (i.e. the tulip form of part A) of the flue gas draft is 23 kW/l.

An alternative worked example embodiment as in FIG. 4, gives a heat exchanger element with an output of approximately 25 kW. For this exemplary embodiment, the thermal inertia is also only 0.17 kg/kW with a compactness of 5.5 kW/1, resulting in a heat exchanger element of 4.3 kg and a volume of 4.6 l.

FIG. 8 is an optical microscopy picture of a strut of an open cell aluminium foam 15 made of aluminium alloy AlSi7 embedded in the co-cast material 16 of the heat exchanger element made of aluminium alloy AlSi10. This body was sand-cast by the method as described above. It clearly shows that the strut's integrity was not altered by the hot melt of aluminium alloy that was cast onto this strut. This type of metallic connection gives heat transfer data which are comparable or better to heat transfer data obtained with a sinter bonding method.

FIG. 9 is an alternative example embodiment of the present invention. It shows a heat exchanger element comprising four flue gas drafts 7, which are water cooled by the water-carrying channel 30. Again, one can identify three distinctive parts in the flue gas draft. Part A comprising the large fins, part B comprising the pins and part C comprising an aluminium porous structure.

The flow system of the water carrying channel in FIG. 12 is also considered to be a parallel connection. Water coming from a central heating pipe system enters the heat exchanger adjacent its bottom side at the location of arrow 110. From here, the water enters the feed-in part 30 a of the water carrying channel. The channel 30 a divides into five separate channel parts 30 b, 30 c, 30 e, 30 f and 30 g. The water divides in these channels, after which the water flows into the common channel 30 d, thereafter the water leaves the heat exchanger via outlet 120. With such embodiment of the water-carrying channel 30, it is effected that only the long walls 13 of the flue gas drafts 7 are water-cooled. Because of the dimensioning of the heat exchanger element 10 resulting in a very flat heat exchanger element, and the heat exchanger element being cooled in a very efficient manner, it does not need cooling on the short walls 14 of the heat exchanger element and makes the heat exchanger element very compact. The optimal heat transfer to the water to be heated, makes that the heat exchanger element nowhere becomes overheated, thus an optimal efficiency is obtained and all parts of the heat exchanger remain sufficiently cooled. Preferably, the parallel channels of the water carrying channel are provided with surface enlarging pins on their inner side, for further enhancing the heat transfer from the metal of the heat exchanger element to the water to be heated. 

1. A heat exchanger element (1) comprising walls (2) from substantially aluminium, said walls (2) enclosing at least one water carrying channel (3) and having at least one flue gas draft (7), at least one wall forming a boundary between said water carrying channel (3) and said flue gas draft (7), said at least one wall being provided with fins and/or pins (8,9) which enlarge the heat-exchanging surface and which extend in the flue gas draft (7), characterised in that said heat exchanger element further comprises a porous metallic body (10) from substantially aluminium, said porous metallic body being placed downstream said heat exchanging surface enlarging pins and/or fins (8, 9) in the direction of the flue gas flow, said walls being cast around said porous body to form a co-cast heat-exchanger element.
 2. A heat exchanger element (1) according to claim 1, wherein the porous metallic body (10) is an open cell metallic foam.
 3. A heat exchanger element (1) according to claim 1, wherein the porous metallic body (10) is a metallic spacer material.
 4. A heat exchanger element (1) according to claim 1, wherein at least one cross sectional surface of said pin and/or fin is smaller than 25 mm².
 5. A heat exchanger element (1) according to claim 1, wherein said water carrying channel comprises a parallel path with respect to the flue gas draft.
 6. Process for the production of a heat exchanger element for a boiler containing a metallic porous body, said process comprising the steps of: a) providing a metallic porous body b) putting said metallic porous body in a core box; c) closing said core box; d) blowing a mixture of sand and binder into the void space in the core box, thereby obtaining a hybrid structure of metallic porous body filled with said sand-binder mix, e) hardening said sand-binder mix thereby obtaining a metallic porous body—sand core; f) removing the core box; g) integrating said metallic porous body—sand core in a flue gas draft sand core; h) placing said flue gas draft sand core in a moulding box together with a water side core; i) pouring molten metal into said moulding; j) cooling of the cast workpiece; k) removing the sand cores.
 7. Process for the production of a heat exchanger element for a boiler containing a metallic porous body, said process comprising the steps of: a) providing a metallic porous body b) putting said metallic porous body in a flue gas draft-core box; c) closing said flue gas draft core box; d) blowing a mixture of sand and binder into the void space in the core box, thereby obtaining a hybrid structure of metallic porous body filled with said sand-binder mix, e) hardening said sand-binder mix thereby obtaining a metallic porous body—sand core; f) removing the core box; g) placing said flue gas draft sand core in a moulding box together with a water side core; h) pouring molten metal into said moulding; i) cooling of the cast workpiece; j) removing the sand core.
 8. Process for the production of a heat exchanger element for a boiler containing a metallic porous body via lost foam investment casting, said process comprising the steps of: a) providing a metallic porous body b) putting said metallic porous body in a flue gas draft-core box; c) closing said flue gas draft core box; d) blowing a mixture of sand and binder into the void space in the core box, thereby obtaining a hybrid structure of metallic porous body filled with said sand-binder mix, e) hardening said sand-binder mix thereby obtaining a metallic porous body—sand core; f) removing the core box; g) building in said metallic porous body sand core into the polystyrene pattern of the heat exchanger element; h) coating of the polystyrene pattern—metallic porous body hybrid cluster with ceramic; i) drying the ceramic coating; j) placing said polystyrene pattern—metallic porous body hybrid cluster into a casting flask and backing up said cluster with un-bonded sand; k) performing mold compaction; l) pouring the polystyrene pattern with the molten metal; m) cooling of the cast workpiece; n) removing the sand cores.
 9. Process according to claim 6, wherein said metallic porous body is a metallic foam.
 10. Process according to claim 6, wherein said metallic porous body is a metallic spacer material.
 11. Process according to claim 6, wherein after step e) said sand core is removed from the periphery of said hybrid structure, thereby obtaining a small border of only metal porous body struts.
 12. Process according to claim 6, wherein the metal is aluminium or an aluminium-alloy.
 13. A heat exchanger element obtained by the methods as in claim
 6. 14. A heating boiler provided with a heat exchanger element according to claim
 1. 